This proposal is concerned with the long-term physiological study of individual mammalian cerebral axons. A method will be explored and developed which, in pilot work, has enabled the applicant to examine impulse conduction properties of central axons in the rabbit, continuously, for as long as 165 days. This period of time constitutes a significant portion of the lifespan of this mammal. The basic strategy entails the use of chronic extracellular recording from the soma of callosal efferent neurons in conjunction with antidromic activation of the efferent axon at one or several points along the axonal path. Impulse conduction properties to be studied over time include axonal conduction velocity, excitability, refractory period, minimal interspike interval, the supernormal period and the subnormal period. In some of the axons studied in pilot work, consistent long term changes in some of the above conduction properties were observed. The causes of such long term fluctuations will be explored. In addition to the elucidation of the normative variations in conduction properties which occur during the mammalian lifespan, the successful development of the proposed method will enable the study of the long-term effects of pharmacological and toxicological agents on impulse conduction properties of aging cerebral axons.